A very marked change in ozone content in upper troposphere and lower stratosphere, observed during international intercomparisons of ozone sondes at Arosa in April 1962, is studied in relation to the synoptic situation. Some values are given for the observed ozone variations with time and space. The potential value of ozone data in synoptic research is discussed.DOI: 10.1111/j.2153-3490.1965.tb01410.x